Why won't the car start and the engine fault light is on?

[Evelyn]

If the car won't start and the engine fault light is on, it may be caused by an oxygen sensor failure or a mass air flow sensor failure. Oxygen sensor failure: Modern vehicles are equipped with two oxygen sensors, one before and one after the three-way catalytic converter. The front oxygen sensor detects the air-fuel ratio under different engine operating conditions, while the ECU adjusts the fuel injection quantity and calculates the ignition timing based on this signal. The rear oxygen sensor mainly monitors the performance of the three-way catalytic converter. Therefore, if the oxygen sensor is damaged or the sensor connector is damaged or loose, it can cause the air-fuel mixture to become too lean or too rich, triggering the fault light. Mass air flow sensor failure: The mass air flow sensor, also known as the air flow meter, measures the amount of air intake and converts it into an electrical signal sent to the ECU. Based on the optimal air-fuel ratio, the ECU determines the amount of fuel to inject. If the mass air flow sensor or its circuit malfunctions, the ECU will not receive the correct air intake signal, leading to improper fuel control and causing the air-fuel mixture to become too lean or too rich, which prevents the engine from operating normally.